1. Field of the Invention
The present invention relates to radio communication systems, and particularly to cellular radio communication system using aircraft.
2. Description of the Related Art
In the United States and elsewhere, domestic cellular telephony coverage is not universally available. Estimates suggest that coverage by terrestrial cellular systems using analog (first generation or 1G) technology now extends to 70% of the co-terminus United States land mass and 95% of the population. Likewise, coverage by terrestrial systems using digital (second generation or 2G/2.5G) technology extends to only 20% of the co-terminus United States land mass but includes 80% of the population.
Within those geographical areas already covered by terrestrial cellular service, vagaries of electromagnetic propagation cause gaps in service with estimates indicating that these gaps are as extensive as 10% to 20% of nominal coverage. As it requires a significant investment, further expansion of the existing terrestrial cellular infrastructure to accommodate the population not presently served is unlikely.
Furthermore, when deployed in terrestrial cellular systems, anticipated third generation (3G) technology will likely cover a co-terminus United States land mass no larger than its immediate 2G/2.5G predecessor. Some authors refer to these shortfalls in terrestrial cellular coverage as the cellular divide or, for generations 2G and beyond, the digital divide.
Prior inventors have brought forth a variety of approaches for providing coverage of mobile telephony users in areas not presently served. Most well known are low earth-orbiting (LEO) satellite-based systems such as Iridium and Globalstar. These systems rely on a constellation of LEO satellites to provide coverage for properly equipped users. The users communicate directly with a satellite, which uses feeder links to relay user traffic to a ground station, or ground stations, for connection with the terrestrial telecommunications infrastructure. Low earth orbiting satellites necessarily provide global or near-global coverage so that the associated investment yields an overall capacity, which greatly exceeds the capacity available for domestic (US) usage at any time.
Also presently in service are several geosynchronous (GEO) satellite-based systems, e.g. MSV, Thuraya and ACeS. These systems place a dedicated satellite (or satellites) above the geographic region to be covered so that their entire capacity is available to properly equipped users therein. Users again communicate directly with the satellite, which relays traffic to a ground station using feeder links.
For either LEO or GEO satellite-based systems, some user equipment also operates with selected terrestrial cellular systems. However, not all user equipment supports dual mode operation, e.g., MSV offers only dedicated single mode user equipment.
Alternate satellite-based systems are also possible. For example, U.S. Pat. No. 5,722,042, which is incorporated by reference herein, advocates a satellite communications system with a double-layered earth-orbiting constellation with a lower first orbit altitude and a higher second orbit altitude. None of these alternate systems appears to have entered service.
Regardless of their orbital characteristics, satellite-based systems opt to impose a fixed set of cells on the earth's surface. This approach forces non-stationary satellites to adjust their antennas to track a coverage area as they move along their orbits and to rapidly switch their antennas between coverage areas periodically to support a new coverage area on the earth's surface. Supporting such fixed patterns simplifies user operation but complicates antenna design and increases satellite cost.
High costs associated with rocket launches make satellite-based systems very expensive to deploy. Moreover, high usage costs and bulky, expensive user terminals limit adoption of deployed satellite-based systems by potential subscribers. Several of these systems have gone through bankruptcy whereas others only recently entered service with financial results still indeterminate.
Other systems for providing coverage of domestic mobile telephony users in unserved areas include a variety of elevated platforms, including an interesting dual use of National Weather Service balloons.
U.S. Pat. No. 3,742,358, which is incorporated by reference herein, and other patents cited therein illustrate the distant origins of knowledge of extensive coverage associated with elevated platforms, e.g., airborne platforms. Subsequently, U.S. Pat. Nos. 4,704,732 and 5,104,059, which are both incorporated by reference herein, identify communications as one application of freely suspended, long endurance high altitude platforms. U.S. Pat. Nos. 4,476,576 and 4,903,036, which are both incorporated by reference herein, employ a tethered aerostat specifically as an antenna to support VLF communications.
U.S. Pat. Nos. 5,949,766 and 6,151,308, which are both incorporated by reference herein, describe ground devices and an elevated wireless communications hub capable of switching, i.e., separating signals from multiple sources and sending them to multiple destinations. U.S. Pat. No. 5,963,877, which is incorporated by reference herein, extends this concept to high altitude platforms that employ antennas capable of creating a cell structure on the earth's surface to support wireless communications including cellular telephony. U.S. Pat. No. 6,061,562, which is incorporated by reference herein, further extends this concept to include a dedicated aircraft flying above the service region while U.S. Pat. No. 6,167,263, also incorporated by reference herein, uses a plurality of dedicated aerial platforms or vehicles, capable of communicating with each other, to provide a global communications network. Finally, U.S. Pat. No. 6,324,398, which is incorporated by reference herein, explicitly emulates the terrestrial cellular infrastructure with ground-based switching centers supporting base stations located on a plurality of dedicated airborne platforms.
All of the foregoing systems strive to keep their elevated platforms stationary over a fixed geographic area to support a fixed service area or set of fixed service areas. In line with this objective, powered elevated platforms rely on tracking antennas much like low earth orbiting satellites but they do not utilize handover of coverage areas as these satellites do.
At present, none of these systems related to elevated, but non-orbiting, platforms has entered operational service. As revenue that they generate must cover all operating costs, nationwide deployment of a cellular-type system comprising dedicated elevated platforms appears unlikely. As intended applications, systems based on elevated platforms most often describe either supplemental cellular coverage in regions with heavily utilized terrestrial cellular infrastructures or prime coverage in heavily populated areas with limited or no terrestrial cellular infrastructure. They do not discuss sparsely populated regions because operation therein does not admit recovery of said platform's operating costs.
Commercial aircraft offer another set of elevated platforms that can provide wireless communications to remote terrestrial users. Since these platforms are airborne and, hence, positioned to offer wireless communications services, for the primary purpose of transporting passengers or freight, wireless communications services must defray only a minor part of an commercial aircraft's operating cost. In addition, commercial aircraft pass over remote areas even though they fly between population centers. With more than 1500 commercial aircraft simultaneously airborne for more than sixteen hours daily, domestic coverage provided by this fleet is extensive.
A few years ago, many commercial airlines began offering airborne (on-board) telephone services by implementing telephone units at specific locations within the cabin of the commercial aircraft, typically placed in seatbacks. This service used UHF frequency bands to link outbound calls (from passengers) to ground stations but, due to high usage costs, never achieved financial success. Such services invariably do not support inbound calls because ground-based callers encounter prohibitive difficulties in identifying a ground station within transmission range of a particular aircraft. Because of disappointing financial results, some airlines are now removing this equipment to avoid the cost associated with transporting its weight.
Several subsequent inventions address shortcomings of airborne telephone services. For example, U.S. Pat. No. 5,651,050, which is incorporated by reference herein, describes a method for directing calls of terrestrial origin to an on-board telephone or telephones without knowing aircraft location. The on-board telephones are dedicated to the aircraft but may be temporarily assigned to passengers using traveler identification numbers.
U.S. Pat. No. 6,052,604, which is incorporated by reference herein, extends this calling method to allow passengers to use their own subscriber identity module (SIM) cards as identifiers while sharing on-board telephone resources. This method invokes the system security associated with SIM cards without requiring dedicated telephone units for each passenger who wishes to avail themselves of outbound and/or incoming calling services.
Although Global System for Mobile Communications (GSM) communications equipment employs SIM cards, neither TDMA nor CDMA equipment do. Re-use of passenger equipment offers a powerful incentive to adoption of airborne telephony service, however. Thus, U.S. Pat. No. 6,249,913, which is incorporated by reference herein, describes a method to use passengers' personal terrestrial cellular telephones with docking cradles that disable on-the-air transmissions from these units within the aircraft. Contrariwise, U.S. Pat. No. 6,249,243, which is incorporated by reference herein, describes a method for using low power, on-the-air transmissions to and from passengers' personal cellular telephones within the aircraft.
U.S. Pat. No. 6,393,281, which is incorporated by reference herein, describes a means for seamless handoff of calls as an aircraft passes out of the coverage area of one ground station and into the coverage area of another ground station.
Although focused on providing broadband services to passengers, U.S. Pat. No. 6,285,878, which is incorporated by reference herein, initially recognized the feasibility of extending these airborne services beyond passengers to include terrestrial users located within line-of-sight of the host aircraft. The method described in this patent is more restrictive than that described herein because it relies on crosslinks between commercial aircraft and requires control over aircraft scheduling to ensure availability of platforms for relaying communications traffic.
In addition, crosslink equipment is expensive especially as it requires pointing and supports multiple levels of relay, i.e., communications traffic from multiple aircraft. Airlines set their schedules based on attracting passengers within constraints imposed by their specific gate assignments at particular airports, which may not readily support long strings of aircraft relaying communications traffic as, for instance, those crossing the North Atlantic Ocean.
Although others have observed that commercial aircraft and cellular telephony make a potent combination, none have addressed coverage gaps that inevitably develop in terrestrial cellular patterns hosted by commercial aircraft. See e.g., V. Pandiarajan and L. Joiner, “Undedicated HAAP Based Architecture for Cellular Data Transfers,” IEEE Southeastcon, pp. 23-26, 2000, which is incorporated by reference herein. As these aircraft choose their own schedules and flight dynamics while operating in an environment that sometimes disrupts both schedules and/or flight dynamics, using commercial aircraft to offer cost-effective wireless communications services requires techniques described in the present invention.